His Masterness is the leader of 4U City.
During most of the 4U City storyarc, nothing is known about His Masterness. No one seems to know who he is or even what he looks like. Alt-Torg speculates that he is that dimension's Dr. Schlock, who controlled 4U-Corp when 4U City was built, but alt-Torg doesn't have any evidence. On the other hand Riff speculates that His Masterness is this dimensions Riff, and they both ridicule the other's suspicions.
It is mostly believed that his Masterness spends all of his time holed up at the top of the Central Tower and rarely, if ever, comes down. Still, Riff believes that he hides somewhere else, away from such an obvious hideout and away from people.
He seems to be terrified of his dimension's Riff, or any dimension's Riff and has issued a standing order for anybody with Riff's DNA to be terminated upon sight. Similarly anyone with Zoë's DNA was to be taken into custody upon identification. When Zoë from Sluggy Prime arrived she was immediately taken to a private medical room in His Masterness' levels. It is unknown what he wants with Zoë or why he is keeping her alive in a condition Executive Rammer describe as never-ending pain. His Masterness also used Riff's DFA to rid the world of everyone who wasn't in 4U City at the time of its creation. Why he would want to do this is not known.
SPOILER WARNING: 4U City Red
This section blatantly gives away the mysteries and/or the ending of the story 4U City Red. If you haven't read 4U City Red, then dude, before you go any further, go read it! This stuff will still be here when you're done.
You have been warned...
His hiding place was eventually discovered by Riff from the Dimension of Rain, who had spent 30 years getting back to his home dimension after Schlock's treachery. Riff discovered that the dimensions boundaries were seriously wounded by the intense DFA activity following the R&D Wars, choosing to work with Schlock in repairing that damage instead of acting out his revenge.
Eventually Schlock died of old age, but his prior arrangements for his own death led Executive Rammer, who was secretly Schlock's biological son, to believe that his father was murdered, and that the unknown killer would now be the new "His Masterness". Rammers reason for initially assisting Riff in his battle against His Masterness was the hope that Riff would eventually lead him to Schlock's killer.
A last relevation was that 4U city had been operated by the city artificial intelligence from before "our" Riff arrived. The command to kill Riff and rescue Zoë was set up by Schlock and never disabled. And the decision to replace the human squads of REA-5 with robotic REA-6 units were taken by the AI, that also chose to respond to the revolution with excessive force.